[Ford Man]

The upstream 02 sensor is one of several sensors that controls the air/fuel mixture and has nothing to do with the PO420. The downstream 02 sensor takes a reading of the pollutants post catalytic converter telling the ECU the efficiency of the converter. If it's throwing this code usually it's a bad converter. Was the check engine light on when you had the emissions test? Probably not unless they just do a tailpipe test and it passed by the skin of it's teeth. If the emissions testing equipment is hooked into the OBD II connector and the CEL was on the ECU would have sent a message to the testing equipment telling it there was a problem and the car would have failed emissions testing. If the light just came on after the emissions test then the converter was probably removing enough of the pollutants at the time to barely meet the standard, but the converter efficiency dropped shortly afterward turning on the CEL. Unhooking the battery for a few seconds will turn the light off, but if it's a problem with the system it will probably be back on within 50-150 miles and if they hook up to the OBD II port when doing the emissions test and the battery has just been disconnected the ECU will also tell the testing equipment that all the systems aren't ready for emissions testing which usually takes 50+ miles for everything to recalibrate. I agree that the oil burning could likely be what is causing the problem just as explained in one of the previous posts. Also on OBD II systems if the converter is working correctly for a certain number of starts while the CEL is on the light will go out by itself without having to disconnect the battery or erase the ECU's memory with a code reader. This is not only true with this code, but any code.